The present invention relates to a control apparatus for accurately controlling a plant by using a delta-sigma (ΔΣ) modulation algorithm.
A method for controlling a plant (controlled object) by using a delta-sigma modulation algorithm (or a sigma-delta (ΣΔ) modulation algorithm or a delta (Δ) modulation algorithm) is known, as shown in the Japanese Patent Publication No. 2003-195908. As long as the plant is capable of generating an appropriate control output in response to a control input having an on/off switching behavior, the plant can be accurately controlled by the delta-sigma modulation algorithm.
FIG. 15 is a block diagram showing an example of a control scheme using a delta-sigma modulation algorithm. A controller 101 calculates a manipulated variable for causing a controlled variable of a plant to converge to a desired value. A modulator 102 uses the delta-sigma modulation algorithm to modulate the manipulated variable. The modulated manipulated variable is input into the plant 103. An output from the plant 103, which is the controlled variable, is fed back to the controller 101.
FIG. 16 shows exemplary behaviors of various signals in accordance with a control scheme using a conventional delta-sigma modulation algorithm. Rcain indicates the manipulated variable calculated by the controller 101. Vcain indicates the modulated manipulated variable, which is a modulated signal generated by the modulator 102. CAIN indicates an output from the plant 103, which is a controlled variable. CAIN_cmd indicates a desired value of the controlled variable. The modulated signal Vcain is generated to switch between +d and −d with respect to a predetermined center value. The amplitude of the modulated signal Vcain is 2d. When the manipulated variable Rcain is within the amplitude 2d of the modulated signal, the delta-sigma modulation algorithm is capable of reconstructing the manipulated variable Rcain as the modulated signal Vcain. During a period from time t0 to time t1, an appropriate modulated signal Vcain is generated because the magnitude of the manipulated variable Rcain is smaller than the amplitude 2d of the modulated signal. Thus, the controlled variable CAIN can be appropriately controlled to follow the desired value CAIN_cmd by applying the modulated signal Vcain to the plant.
However, when the magnitude of the manipulated variable Rcain exceeds the amplitude 2d of the modulated signal as shown from time t1, such increased manipulated variable Rcain cannot be appropriately modulated because the amplitude of the modulated signal is limited to 2d. A portion of the manipulated variable exceeding the amplitude 2d is not reflected in the modulated signal Vcain. As a result, the manipulated variable Rcain is substantially limited as shown by a dotted line 105. Since the modulated signal is generated with a portion of the manipulated variable Rcain being lost, a deviation occurs between the controlled variable CAIN and the desired value CAIN_cmd. Thus, from time t1, the plant cannot be appropriately controlled.
Such condition may occur, for example, in a phase control for a camshaft of an engine of a vehicle. When the phase of the camshaft is controlled by an actuator, that actuator is regarded as a plant. If the torque generated by the actuator decreases due to heat generated by the actuator, or if the friction characteristics of the actuator vary due to variations and age deterioration of the actuator, such condition may occur.
In order to handle such condition, there is considered a method for calculating in advance a maximum value and a minimum value which the manipulated variable can take. The amplitude 2d of the modulated signal is established to contain the maximum and minimum values. According to such a method, however, the amplitude of the modulated signal may increase. Such increase of the amplitude may destabilize the output of the plant.
An electromagnet may be used as the actuator. The electric current of the electromagnet decreases as the resistance of the electromagnet increases. This decrease of the electric current reduces the torque generated by the actuator. In order to avoid such decrease of the torque, there is considered a method for feedback controlling the electric current of the electromagnet and generating the torque corresponding to the manipulated variable determined by the feedback control. According to this method, the controlled variable can converge to a desired value even when some variations occur in the torque due to the electric current. However, such an electric current control cannot handle changes in the friction characteristics.
Thus, there is a need for an apparatus or method for implementing a delta-sigma modulation that is capable of generating a modulated signal adapted to changes in the manipulated variable.